A conventional developing cartridge mounted in an image forming device includes a toner hopper, a supply roller and a developing roller. Toner accommodated in the toner hopper is supplied to the supply roller and then to the developing roller. The developing roller and the supply roller are respectively provided with a drive gear, while the developing cartridge itself is provided with an input gear to which driving force from a motor of the image forming device is transmitted via a coupling member. The input gear is meshingly engaged with each of the drive gears. With this configuration, the driving force from the motor is transmitted to both drive gears simultaneously via the coupling member and the input gear, thereby rotating the supply roller and the developing roller.